How would you regulate that? By just simply looking at them? If they're fat, they get taxed more? What if they had an out of control thyroid problem? Would that exempt them from the tax? What if they had polycystic ovarian syndrome? Would their doctor have to submit paperwork proving there was an actual reason why they couldn't lose weight?
In the society that we live in now, with no one being held accountable for their actions, if you attempt to enact such a tax, they would be a mass exodus to their doctor's office to get their exemption. After they had gotten their exemptions, they would be flooding the media about how unfair this is and how they need a safe zone to talk about their issues.
So, you can't tax someone in order to shame them into losing weight. It's not going to work.